High strength composite fibers have been used for a variety of applications. For example, the use of externally bonded fiber reinforced polymer (FRP) sheets, strips, and fabrics have been recently established as an effective tool for rehabilitating and strengthening steel-reinforced concrete structures. Steel-reinforced concrete beams strengthened with FRP strengthening systems show higher ultimate load strengths compared to non-strengthened concrete beams. However, available FRP strengthening systems suffer from a variety of disadvantages and drawbacks including lack of ductility and high orthotropic characteristics.
Loss of beam ductility is partially attributable to the brittle nature of fibers used in FRP strengthening systems. Fibers commonly used in FRP strengthening systems, such as carbon fibers, glass fibers, or aramid fibers while exhibiting higher ultimate tensile strengths than steel reinforcement, tend to fail catastrophically and without visual warning. Visual indicators of structural weaknesses are desirable as they permit the opportunity for remedial actions prior to failure. Accordingly, it would be desirable to realize the strengthening benefits of FRP systems without sacrificing beam ductility.
As to the timing of the load gains from FRP strengthening, it is noted that FRP strengthening materials behave differently from steel. Although fibers used in FRP materials have high strengths, they generally stretch to relatively high strain values before providing their full strength. Steel also has a relatively low yield strain value (on the order of 0.2% for Grade 60 steel) compared to the yield strain of commonly used FRP fibers (on the order of 1.4-1.7% for Carbon fibers and 2-3% for glass fibers). Accordingly, the degrees of contribution of the reinforcing steel and the strengthening FRP materials differ with the magnitude that the strengthened element deforms, with FRP contributions being most significant after the yield strain of steel. Stated differently, the steel reinforcement commonly yields before the FRP provides any significant strengthening. As the working or design load of a structural component is principally based upon its yield strength, the fact that currently available FRP strengthening systems contribute a majority of the gained increase in load capacity after, rather than before or simultaneously with, the yielding of the steel reinforcement limits the usefulness of FRP strengthening systems.
In attempting to provide reasonable contribution from FRP material during limited deformations, some designers have increased the cross-sectional area of the FRP sheets. However, this approach is not economical. Moreover, the added cross-sectional area makes debonding of the FRP strengthening material from the surface of the concrete/steel beam more likely due to higher stress concentrations, thereby increasing the probability of undesirable brittle failures. Other approaches to more fully capitalizing on the strength of FRP fabrics have focused on the use of special low strain fibers, such as ultra high modulus carbon fibers. While this approach does improve the contribution of the FRP strengthening prior to yielding of the steel reinforcement, the fibers still contribute to brittle failures.
Additionally, currently available FRP fabrics, sheets, and strips also have high orthotropic characteristics. That is, the fabrics provide strengthening only in the direction of fiber orientation. The orthotropic characteristic of FRP fabrics limit their usefulness in applications subjected to multi-directional loads such as simultaneous flexure and shear strengthening of structural components.
In view of these deficiencies in the art, there is a need for a ductile structural fabric, such as an FRP fabric or sheet. In certain applications, such as the strengthening of steel-reinforced concrete beams or structural components, the fabric also preferably exhibits a low strain yield so that the fabric effectively enhances the strength of the beam prior to yielding of the steel reinforcement. Additionally, there is also a desire to provide a ductile structural fabric which can be used for strengthening in more than one direction. In other words, the fabric is desired to have reduced orthotropic characteristics.
The present invention is directed to a structural fabric having a first fiber with a first ultimate strain, a second fiber with a second ultimate strain greater than the first ultimate strain, the first and second fibers being in the same plane. The invention is further directed to a structural fabric having a plurality of axial fibers and a plurality of first diagonal fibers braided with the axial fibers and oriented at a first braid angle relative thereto. The axial fibers include first and second fibers each with an ultimate strain. The ultimate strain of the second fiber again being greater than the ultimate strain of the first fiber. Additionally, the invention is directed to a concrete beam strengthened with the structural fibers of the present invention.
Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.